Increasingly, the drilling of oil and gas wells is no longer a matter of drilling vertical bore holes from the surface to a zone of hydrocarbon recovery using a bit attached to the bottom of discrete rotatable lengths of drill string. Technology and techniques have been developed to deviate the bore's trajectory at angles of up to and sometimes exceeding 90.degree. from the vertical. In this way, significant economic zone enhancement can be achieved for example by creating a bore that actually follows an oil or gas bearing strata.
Unlike most vertical wells that are normally lined with casing to well bottom, horizontal bores are sometimes cased to just above the kickoff from the vertical section with the remainder of the well comprising unlined open hole formation. This poses well stimulation problems particularly if conventional acidizing or bleaching techniques are ineffective or inappropriate having regard to formation and formation fluid characteristics. Skin damage to the formation surrounding the horizontal or vertical bore can occur for example as a result of having been drilled with a polymer-based mud system, carbonate formations in particular being susceptible to contamination by polymers used in some drilling muds. Normal practice for skin damaged carbonate involves acid or bleach treatment. Bleach can be ineffective with respect to some polymers and acid washes, due to their depth of penetration, can open fractures to nearby water bearing formations.
Economic considerations also arise in that not all of the horizontal (or vertical) section penetrated by the bore will exhibit viable productivity traits. As a result, these sections are not economically susceptible of stimulation. Selective treatment of only portions of the bore will be preferred in such cases. Inflatable packers can be used to isolate portions for selective stimulation but sealing against an open bore is less reliable than sealing off sections of steel casing, nor is this approach conducive to controlling penetration rates beyond desirable limits.
Aromatic acids to break down paraffins and asphaltenes and an underbalanced acid wash and squeeze reduce penetration depths and reduce acid volumes required. However, for selective stimulation, packoffs will be required.